Lacquers and enamels are added today to the greatest extent to protect different surfaces, to improve the appearance and to fulfill specific color psychological and physical-industrial functions, e.g., as warning colors, electrically conducting, IR-reflecting and sunlight absorbing coatings.
By the term enamels there is understood solutions, suspensions or dispersions of polymers or polymer forming materials treated with pigments, e.g., with carbon black in which there occurs forming the lacquer film by physically drying, i.e., by evaporation of the solvent or forming a film on the surface of objects to be coated by oxidation or reaction.
Important binders for the different lacquer systems are for example alkyd resins, melamine resins (e.g., melamine-formaldehyde resins), polyol-polyisocyanate (polyurethanes), polyacrylate resins, epoxide resins, chlorinated rubber, etc.
Depending on the intended area of use, different specific requirements are placed on the appearance and the resistance of the coatings. The binder and the color imparting pigments must be correct for the requirements placed thereon. In only a few cases are individual pigments added for coloration, e.g., for black coatings carbon black, for white coatings titanium dioxide, however, predominantly there are added mixtures of pigments in order to produce the desired color tones which are infinitely numerous.
For the production of black lacquers for many years there has been approved carbon black. This black pigment in contrast to all competitive products has the advantage of high depth of color and very good light stability. Deep black lacquers are produced today with specially adjusted high value color blacks which have the following physical-chemical characteristics:
______________________________________ Primary particle size determined electron- microscopically nm 10-25 BET surface area m.sup.2 /g 100-800 Volatile content % 5-20 pH value 3-5 ______________________________________
Typical members of this class are, for example, the color blacks FW 1.RTM., FW 200.RTM., FW 2.RTM., S 170.RTM., S 160.RTM., Spezialschwarz 5.RTM. and Spezialschwarz 4.RTM. of DEGUSSA.
Besides their use for the production of black full tone paints carbon black is of great significance as a partial tone pigment. Carbon black finds use not only for gray shades in various steps of lightness and nuances but also in establishing the gray value of multi-color tones. For this use, however, there are considerable difficulties with the precedingly mentioned carbon blacks. Because of the finely divided state of these carbon blacks there are produced, e.g., with mixing with white pigments only brownish color tones.
To overcome the named disadvantages coarse particle carbon blacks were added which could be produced according to the lamp black process and the furnace black process. As coarse particle in this connection there is understood carbon blacks whose electron microscopically determined particle size is greater than 35 nm and which have an iodine adsorption according to DIN 53582 (German Industrial Standard 53582) of less than 60 mg/g. These carbon blacks to be sure have less strength of color but in mixing with the white material show the preferred bluish color tone. However, they have the undesired property of flushing out in pigment mixtures. By "flushing out" there is understood that after the application of the color varnish (or color lacquer) there occurs a gradual distinct separation of the, e.g., white base pigment (predominantly titanium dioxide) and the black shading pigment. The separation can be expressed in such a manner that the carbon black flushes out in horizontal and/or vertical direction. This phenomenon is very undesired since through this the reproducibility of the desired color tone is made difficult. The basis for the flushing out is frequently sought for in the very different relative sizes of the white pigment (titanium dioxide) and the shading pigment (carbon black).
Titanium dioxide is much coarser than the coarsest carbon black. Also charging ratios are made responsible for the flushing out. There are not attempts to keep the flushing out effect within limits by using various procedures, e.g., by using wetting agents and by increase of the structural viscosity of the lacquer. These procedures, however, have had only a limited success.